1. Field of the Invention
The present invention relates to a one-piece golf ball.
2. Description of the Related Art
Usually, a one-piece golf ball is used in a golf range or a like site, and a coat is formed on the golf ball surface for improving external appearance with luster being applied thereto or for protecting a mark or a golf ball body itself. Long-time use of the one-piece golf ball in a golf range may cause discoloration with time by exposure to UV rays (sunlight), wind or rain. Or the coat on the golf ball may be peeled off by repeated exertion of impact to the golf ball or repeated cleaning of the golf ball. Such discoloration or peeling-off of the coat is not desirable because the golf ball having such poor external appearance may give impression that the golf ball is degraded. Further, as a result of actual progress of degradation of a one-piece golf ball body, the golf ball may suffer from crack or a like phenomenon.
Under the aforementioned circumstances, one-piece golf balls with improved weatherability and durability have been proposed. For instance, Japanese Unexamined Patent Publication No. 2002-325864 (hereinafter, called as “D1”) discloses a one-piece golf ball which is free of yellowish discoloration with time and can retain its whiteness by addition of titanium oxide and a blue pigment in respective predetermined amounts. Japanese Patent No. 3293679 (hereinafter, called as “D2”) discloses a golf ball, wherein titanium oxide, a blue pigment, and a purple pigment are included in respective predetermined amounts in a rubber composition for composing a one-piece golf ball body, and a clear coat is applied onto the golf ball body. Japanese Unexamined Patent Publication No. 7-51403 (hereinafter, called as “D3”) proposes a one-piece golf ball constructed such that a golf ball body in which titanium oxide, a blue pigment, and a purple pigment are included in predetermined respective amounts is covered with a clear coat containing an oxidation inhibitor and a light stabilizer.
In the one-piece golf ball disclosed in D1, an attempt is made to retain whiteness of the golf ball by increasing the degree of a blue color component in the golf ball body, based on a complementary color relation between blue and yellow, while overcoming degradation of the rubber component of the golf ball body by exposure to UV rays or the like, which may cause yellowish discoloration. The art disclosed in D1 has a limitation in that degradation of the rubber component itself is not suppressed. Accordingly, despite an effort of retaining whiteness, crack may occur in the golf ball by exertion of an impact or a like external force to the golf ball, once degradation of the rubber component progresses. Further, if yellowish discoloration outranges the degree of the blue color component, as a result of progress of degradation of the rubber component, the golf ball cannot retain its whiteness.
Further, it is a general practice to use an organic stabilizer such as an oxidation inhibitor or a UV absorbent, as an ingredient of a coat to be applied on the conventional one-piece golf ball as disclosed in D3. Since such an organic stabilizer decomposes by itself by hydrolysis or attacking of radicals generated by UV rays, the anti-oxidizing or UV absorbing performance cannot be sustained for a long time. If the action of the organic stabilizer ceases or is weakened, it is highly likely that a base resin of the coat may be decomposed by exposure to UV rays, or the degraded coat may be peeled off from the surface of the golf ball body. As a result, the coat may no longer protect the golf ball body, and discoloration or crack may occur by degradation of the rubber component of the golf ball body due to exposure to UV rays, wind, or rain.
D2 utilizes rutile type titanium oxide in an attempt to improve weatherability. Titanium oxide degrades the rubber composition of a one-piece golf ball body by photocatalytic action of the titanium oxide, despite a merit that the titanium oxide acts as a UV absorbent, and contributes to improvement of weatherability.
Specifically, by exposure to UV rays, an electron-hole pair consisting of a free electron (e−) on a conduction band and a positive hole (p+) on a valance electron band is generated in the titanium oxide due to excitation of the electron on the valance electron band, as represented by the following formula (1).TiO2+hr→p++e−  (1)
The positive hole (p+) is entrapped by OH− in adsorbed water on the surface of the titanium oxide, thereby generating an OH free radical, as represented by the formula (3). The generated free radical decomposes a rubber component by its powerful oxidizing action.H2O→H++OH−  (2)OH−+p+→{dot over (O)}H  (3)
The free electron (e−) reduces Ti4+ in a crystal of the titanium oxide to Ti3+ as represented by the formula (4). Subsequently, O2 in the air is adsorbed to the Ti3+, and turns to O−2 as represented by the formula (5). Then, the O−2 is reacted with the H+ in the formula (2), and turns to an H{dot over (O)}2 free radical while undergoing the reaction as represented by the formula (6). Similarly to the {dot over (O)}H free radical, the H{dot over (O)}2 free radical decomposes the rubber component around the titanium oxide by its powerful oxidizing action.[Ti4+]+e−→[Ti3+]  (4)[Ti3+]+O2→[Ti4+]+O−2  (5)O−2+H+→H{dot over (O)}2  (6)
The above photocatalytic action may occur even in use of rutile type titanium oxide. Further, there is a drawback that weatherability of the golf ball body cannot be improved sufficiently, as far as the cover composition contains titanium oxide, despite inclusion of a color stabilizer such as a UV absorber and a light stabilizer, as disclosed in D3.